Advanced optical network technologies such as Dense Wavelength Division Multiplexing (DWDM) form the foundation for fiberoptic telecommunications networks, enabling worldwide traffic aggregation and metro and regional network consolidation. Such optical fiber networks often use reconfigurable optical add-drop modules (ROADMs) to deliver new flexibility to DWDM networks by enabling dynamic, transparent optical wavelength add/drop functioning.
In general networks, the degree (D) of a network node is usually taken to mean a measure of how many network nodes are connected immediately adjacent to that node.
Wavelength selective switch (WSS) technology incorporated in ROADMs may use multi-degree (nD) ROADM architectures with a broadcast-and-select architecture. An optical splitter distributes wavelengths to a ‘drop’ path fixed wavelength demultiplexer and to each express direction. For each outgoing direction, a WSS is used to selectively combine ‘add’ wavelengths from an ‘add’ path fixed-wavelength multiplexer with channels selected from each express direction.
In many current architectures, a particular transmitter can send signals in only one output direction, towards only one adjacent network node. However, a ‘degreeless’ architecture is preferable, in which a particular transmitter can send signals to any direction, that is, to any adjacent node. As the optical fiber networks evolve toward ‘degreeless’ architectures, many implementations of these architectures require a M×N WSS function, as described by Peter Roorda and Brandon Collings (“Evolution to Colorless and Directionless ROADM Architectures”, OFC 2008, paper NWE2). WSS technology is well-suited to extending ROADMs to allow automated assignment of the add/drop wavelength, a functionality often referred to as colorless switching. Colorless ROADM architectures address the full automation of wavelength assignment, but the outbound direction of the transponders remains fixed.
For example, a power splitter may be used to broadcast the ‘add’ wavelengths to a WSS for each direction, and another WSS is used to select the direction for the associated ‘drop’ wavelength. Using this architecture with a colorless MUX/DEMUX and amplifiers for compensating insertion loss, an ‘add/drop’ port can be assigned to any wavelength and coupled to any direction in a fully automated fashion.
A basic simplified structure of a 1×N WSS using arrays of adjustable reflectors. The adjustable reflectors may be MEMS mirrors that can be tilted in 1-dimension about one axis, as shown in top view in FIG. 1a. This has been described by Ducellier et al. in U.S. Pat. No. 6,707,959 issued Mar. 16, 2004, which is incorporated herein by reference. Port switching in the plane of the drawing is effected by suitably tilting MEMS mirrors of a modifying (MEMS) array about an axis.
In this example according to prior art, a basic simplified wavelength switching module 102A comprises a light redirecting element, such as a spherical reflector 120, used to receive a beam of light comprising wavelength multiplexed signals from a front-end unit 122 and to re-image the beam onto a micro-electro-mechanical systems (MEMS) array 126 after reflection off a diffraction grating 124. Due to the optical dispersion of the diffraction grating 124, a separate image is formed on the MEMS mirror array 126 for every wavelength multiplexed signal present in the beam of light.
Each MEMS mirror of the array is arranged so that it reflects the image corresponding to a wavelength multiplexed signal back to the front-end unit 122 via the diffraction grating 124 and the spherical reflector 120. The mirrors are fabricated to enable tilting about an axis perpendicular to the plane of FIG. 1a by means of a suitable controller.
FIG. 1b shows the front end of the WSS of FIG. 1a in greater detail. Four ports can be used for inputs or outputs with MEMS mirrors used for coupling a particular wavelength multiplexed signal between any two ports. For example, this WSS could be used as a 3×1 switch, i.e. with 3 input ports and 1 output port.
In FIG. 1b, an optically equivalent front end of the wavelength switching module 102A of FIG. 1a comprises four input/output ports 132A-D such as optical fibers, each carry wavelength multiplexed signals. The light beams from the optical fibers 132A-D are collimated by lenses 134A-D before passing through a switching lens 136, which converts the spatial separation between the ports 132A-D to an angular separation at an intersection point 150. Since the rest of the optics in the wavelength switching module 102A serve to re-image intersection point 150 onto the MEMS array 126, for the purposes of this description each tilting MEMS mirror corresponding to a particular wavelength multiplexed signal can be considered as being located at the intersection point 150.
Details of the imaging and dispersing optics are well known in the art, for instance as described by Bouevitch et al. in U.S. Pat. No. 6,810,169 issued Oct. 26, 2004, which is incorporated herein by reference.
FIG. 1c illustrates in greater detail such a prior art 2×2 WSS structure based on tilting MEMS mirror arrays in conjunction with optical circulators. Two optical inputs, ‘IN’ 11 and ‘ADD’ 21, carrying wavelength multiplexed signals entering bi-directional ports 31 and 32 through circulators 10 and 20 are focused by lens 35 into beams 41 and 42, respectively, onto an intersection point N. A concave mirror 40 re-images intersection point N via a diffraction grating 50 and transmission path correction element 100 onto MEMS tilting mirrors 61, 62 of a MEMS array 60. After reflection off the MEMS tilting mirrors 61, 62 the beams return by essentially the same route to the intersection point N from where they are collimated into the bi-directional ports 31 and 32, for transmission through circulators 10 and 20 into ‘EXPRESS’ 12 and ‘DROP’ 22 outputs, respectively.
There are essentially two possible choices for the axis about which the MEMS mirrors are tilted: vertical or horizontal. In principle there is no difference between the two, however factors such as optical beam cross-section, spot shape, preferred operating configuration, switch module geometry and similar would in practice determine the choice. Thus FIG. 1b could be regarded as a top view for the case where the MEMS mirrors are tilted about a vertical axis, whereas the same figure can be regarded as side view for the case where the MEMS mirrors are tilted about a horizontal axis. The latter case will be used without loss of generality in the following description with the understanding all the embodiments would be equally functional for the vertical axis case.
FIG. 2a shows a possible way of configuring a M×N WSS from a M×1 WSS and a 1×N WSS. This example illustrates 5 input ports and six output ports forming a 5×6 WSS. In this configuration, the single output of the M×1 WSS and the single input of the 1×N WSS are connected together.
Each wavelength multiplexed signal can be routed independently, however only one instance of each wavelength multiplexed signal can be passed from an input port to an output port. In other words, the configuration exhibits “wavelength blocking”, which poses an obstacle to achieving true arbitrary configurability.
As achieving true arbitrary configurability is considerably more difficult, network designers are likely to accept the wavelength blocking restriction in their designs.
Another potentially more serious problem with the configuration of FIG. 2a is “hitting” during switching operations, which results in unwanted transient signals appearing at the output ports during switching operations.
It is an object of the invention to provide an M×N WSS which performs wavelength switching without “hitting”, i.e. a hitless M×N WSS.
A further object is to provide a hitless M×N WSS that can take advantage of low-cost, manufacturable MEMS mirrors which are tilted only in 1-dimension.
Another object of the invention is to provide a method of operating such M×N WSS in a hitless or quasi-hitless manner.